Process for controlling agglomeration in the manufacture of TiO2

ABSTRACT

A chloride process for producing TiO 2  by addition of an inert gas in a vapor phase reaction of TiCl 4  and an oxygen-containing gas is disclosed.

BACKGROUND OF THE INVENTION

This invention relates to a chloride process for producing TiO₂ byaddition of an inert gas in a vapor phase oxidation of TiCl₄ and moreparticularly, to a process for controlling TiO₂ agglomeration byaddition of the inert gas into a reaction zone where the TiO₂ particlesare formed.

U.S. Pat. No. 4,574,078 and related U.S. Pat. Nos. 4,24 1,042 discloseaddition of an inert gas as a carrier gas and diluent in a process tohydrolyze volatile titanium compounds. Therein, the inert gas is addedbefore reaction of the volatile titanium compound with steam. No mentionis made of using an inert gas to control particle size and/oragglomeration of the TiO₂.

A chloride process for producing TiO₂ pigment by reacting O₂₋ containinggas and TiCI₄ at temperatures ranging from 900° to 1600° C. in a vaporphase is known. The resulting hot gaseous suspension of TiO₂ particlesand free chlorine is discharged from the reactor and must be quicklycooled below 600° C. within about 5 to 100 seconds. This cooling isaccomplished in a conduit, i.e., a flue so that undesired TiO₂ particlesize growth is prevented and particle agglomeration is minimized.Pigment product properties such as carbon black undertone (CBU) andgloss are a function of primary particle size and particleagglomeration, respectively. If high agglomeration of TiO₂ results, theTiO₂ must be milled or ground in an energy intensive, expensive processsuch as micronizing to reduce the size of agglomerates in order toachieve the desired pigment properties.

The chloride process described above, however, produces TiO₂ withvariable agglomeration as production rate changes. The need thereforeexists for a solution to maintain the degree of agglomeration asconstant as possible. The degree of agglomeration changes also accordingto size and design of cooling flues. There is need to maintain a givenlevel of agglomeration if larger diameter flues are used as these mightlead to reduced agglomeration on account of reduced turbulence. Thepresent invention meets those needs.

SUMMARY OF THE INVENTION

In accordance with this invention, there is provided a chloride processfor producing TiO₂ comprising reacting at least vaporous TiCl₄ and anoxygen-containing gas in the presence of an inert gas.

It has been found that the process of this invention maintains thedegree of agglomeration of a TiO₂ pigment as constant as possible thusresulting in a more uniform quality product. The quality of the TiO₂pigment product is linked to the degree of agglomeration. Further,economic benefits in the TiO₂ manufacturing process are obtained. Thisinvention also provides greater flexibility for producing high and lowgloss grades of pigmentary TiO₂ if the size of the reactor and/orcooling flues is increased, and for producing both high and low glossgrades on the same size reactor and cooling flues.

DETAILED DESCRIPTION

The production of TiO₂ by the vapor phase oxidation of TiCl₄ is wellknown and disclosed in U.S. Pat. Nos. 2,488,439 and 2,559,638, theteachings of which are incorporated herein by reference. The presentinvention relates specifically to an improvement in the aforementionedprocesses.

TiCI₄ is evaporated and preheated to temperatures of from about 300° toabout 650° C. and introduced into a reaction zone of a reaction vesselwhich is raised to a peak temperature of about 1000° to 1600° C. atabout 10-80 psig.

The oxygen-containing gas is preheated to at least 1200° C. and iscontinuously introduced into the reaction zone through a separate inletfrom an inlet for the TiCl₄ feed stream. Optionally, theoxygen-containing gas can also contain a vaporized alkali metal saltsuch as CsCl or KCl, etc. which is one of the tools used to controlparticle growth.

As a result of mixing of the reactant streams, substantially completeoxidation of TiCl₄ takes place but for conversion limitations imposed bytemperature and thermochemical equilibrium. Solid particles of TiO₂form. The reaction product containing a suspension of TiO₂ particles ina mixture of chlorine and residual gases is carried from the reactionzone at temperatures considerably in excess of 1000° C. and is subjectedto fast cooling in the flue. The cooling can be accomplished by anyconventional means as known in the art or described below. TiO₂particles leaving the cooling flues are often referred to as an"oxidation base".

In carrying out the invention, the inert gas is added downstream fromthe TiCl₄ stream addition. The exact point of inert gas addition orinjection will depend on the reactor design, flow rate, temperatures,pressures, production rates and rate of cooling of the reaction mass,but can be determined readily by testing to obtain the desired effectson agglomeration and particle size. For example, the inert gas may beadded at one or more points downstream from where the TiCl₄ andoxygen-containing gas are initially contacted. Often, the point orpoints of addition will be located at a downstream distance traveled bythe reactants or reaction products of about 2 to about 200 feet,preferably about 5 to about 50 feet, after the initial contact of thereactants.

Suitable chemically inert gases include nitrogen, chlorine, carbondioxide, mixtures thereof, or gas mixtures rich in nitrogen, chlorine,and/or carbon dioxide, and mixtures thereof, preferably nitrogen of >90%purity. Mixtures rich in a particular inert gas is defined as >75% ofthe inert gas or gases present in the mixture. In a preferredembodiment, nitrogen is added continuously downstream in the conduit orflue where scouring particles or scrubs are added to minimize thebuildup of TiO₂ in the interior of the flue during cooling as describedin greater detail in U.S. Pat. No. 2,721,626, the teachings of which areincorporated herein by reference. In this embodiment the nitrogen can beadded at one or more points either individually or simultaneouslythrough one or more nozzles or orifices. A relatively small amount ofthe chemically inert gas such as cold, high pressure nitrogen isinjected often in amounts from about 0.005 pounds to about 0.5 poundsper pound of TiO₂, preferably about 0.01 pounds to about 0.1 pounds perpound of TiO₂, and the temperature of nitrogen is about -200° to about1000° C., preferably about -20° to about 200° C.

The injection of the chemically inert gas at high pressure controlsagglomeration through turbulent dissipation of kinetic energy. Flowingfrom a high pressure source, the injected gas accelerates to highvelocity as it passes through a nozzle or orifice into the reactionmixture. The high velocity gas has a large kinetic energy per unit mass,equal to one-half of the square of its velocity. The total rate ofaddition of kinetic energy by the injected gas is equal to the kineticenergy per unit mass multiplied by the mass flow rate of injected gas.This kinetic energy is dissipated by turbulence generated as theinjected jet decelerates while mixing with the reaction mixture. Thegenerated turbulence increases the collision rate between particles inthe reaction mixture, and thus increases the degree of agglomeration. Bycontrolling the rate of addition of kinetic energy, the degree ofagglomeration can be controlled.

The rate of addition of kinetic energy is affected by both the mass rateof gas injection and the kinetic energy per unit mass of injected gas,the latter being equal to one-half the square of gas velocity. The sizeand shape of the injection nozzle or orifice, and gas pressure upstreamof the orifice, control the mass rate of injection and the kineticenergy per unit mass. For a given injection nozzle or orifice, both themass flow rate and the jet velocity increase with increasing ratio ofinjection gas pressure upstream of the nozzle or orifice to the reactorpressure. When the pressure ratio is greater than or equal to thatrequired to obtain sonic velocity in the orifice or nozzle throat(section of minimum cross-sectional area), the mass flow rate becomesdirectly proportional to the upstream pressure. In this condition, themaximum exit velocity, and hence kinetic energy, may be obtained using asupersonic nozzle shape with exit area to throat area ratio calculatedto give the maximum exit supersonic velocity corresponding to thepressure ratio. For a given pressure ratio, the mass flow rate isdirectly proportional to the minimum cross-sectional area of theinjection nozzle or orifice. Use of a supersonic nozzle shape, however,is not critical to the invention. Any nozzle shape, with the appropriateminimum cross-sectional area, may be used to carry out the inert gasinjection.

For a given desired rate of kinetic energy addition, use of largernozzles or orifices enables use of lower injection gas supply pressure,but requires more gas, whereas use of smaller nozzles or orifices allowsuse of less injection gas but requires greater supply pressure. Incarrying out the invention, often the injection nozzles or orifices maybe sized to provide the desired gas injection rates as described abovewith supply pressures ranging from about 50 to about 10,000 psig abovereactor pressure, preferably about 500 to about 5000 psig. The rate andpressure of the introduction of the inert gas will vary depending uponthe desired degree of agglomeration of the TiO₂ particles.

The TiO₂ pigment is recovered from the cooled reaction products byconventional separation treatments, including cyclonic or electrostaticseparating media, filtration through porous media or the like. Therecovered TiO₂ may be subjected to surface treatment, milling, grindingor disintegration treatment to obtain the desired level ofagglomeration.

TiO₂ pigment products are tested for Carbon Black Undertone (CBU), ameasure of particle size uniformity that depends to a certain extent onthe amount of particles present as agglomerates. The higher the CBU, thesmaller the particles. A typical CBU for TiO₂ used in paint is about 10.CBU is determined by mulling together a suitable liquid, such as lightcolored oil and standard weights of the sample and a standard carbonblack. The mixture is spread on a panel and the relative blueness of thegray mixtures observed. Fine particles give bluer undertone or higherCBU. CBU is described in greater detail in U.S. Pat. No. 2,488,440, theteachings of which incorporated herein by reference except using areference value of 10 rather than 100 as used therein.

Particle size distribution of the pigment products is measured bysedimentation analysis, with a SEDIGRAPH®, i.e., x-ray sedimentometer(Micromeritics Instrument Corp., Norcross, Ga.) after dispersion inaqueous suspension by fixed level sonication. The percent greater than0.6 microns fraction is a measure of agglomeration and of the potentialfor peak gloss in the finished product, a value that cannot be exceededwhile applying any reasonable grinding energy level. The inert gasinjection can be carried out in a way that affects agglomeration.

To give a clearer understanding of the invention, the following Examplesare construed as illustrative and not limitative of the underlyingprinciples of the invention.

EXAMPLES EXAMPLE 1

TiCl₄ vapor was heated to 415° C. and continuously admitted to areaction chamber. Simultaneously, oxygen, preheated to 1545° C. wascontinuously admitted to the same reaction chamber through a separateinlet. The production rate was 18 tons TiO₂ per hour. The reactantstreams were rapidly mixed. At a location approximately 45 feet from thepoint where the gaseous TiCl₄ and oxygen were initially contacted, about1100 lb/hr of nitrogen at a pressure of 1100 psig at -15° C. wereinjected into the reaction mass through a nozzle for a test period of 90minutes. The gaseous suspension of TiO₂ was then quickly cooled in theflues.

The titanium dioxide pigment was separated from the cooled gaseousproducts by conventional means. The recovered titanium dioxide pigmentwas then treated by conventional pigment treatment procedures and groundto desired texture.

EXAMPLE 2

A test similar to that described in Example 1 was performed. Thetitanium tetrachloride was preheated to 410° C. The oxygen was preheatedto 1540° C. The production rate was 14 tons TiO₂ per hour.

(A) At a location approximately 10 feet from the point where the gaseousTiCl₄ and oxygen were initially contacted, 1600 lb/hr of nitrogen at1600 psig and at -15° C. were injected into the reaction mass.

(B) A second injection of nitrogen, at -15° C., was also performedsubsequent to the first injection at the same location but with 600lb/hr of nitrogen at 600 psig.

EXAMPLE 3

A third series of tests similar to Example 1 was performed. Theproduction rate was 14 tons TiO₂ per hour. The reactant streams wererapidly mixed. At a location approximately 45 feet from the point wherethe gaseous TiCl₄ and oxygen were initially contacted, high pressurenitrogen gas was added to the flue through one or two nozzles. The valvearrangement allowed the use of both nozzles simultaneously orindividually. Nozzle #1 was designed to add 1000 pph of N₂ at 1000 psig.It was located 45'7" from the point where the reactants were initiallycontacted. Nozzle #2 was designed to add 3000 pph of N₂ at 1000 psig. Itwas located 45'10" from the point where the reactants were initiallycontacted. As pressure changes, the N₂ feed rate changes proportionally.

(A) 3800 lb/hr of nitrogen at 950 psig and -25° C. was added to thereaction mass. This was accomplished by adding the nitrogen through bothnozzles simultaneously;

(B) 3150 lb/hr of nitrogen at 1050 psig and -25° C. was added to thereaction mass. This was accomplished using nozzle #2 only; and

(C) 1350 lb/hr of nitrogen at 1350 psig and -25° C. was added to thereaction mass. This was accomplished by using nozzle #1 only.

Evaluations

The quality of the products of the examples were evaluated usingconventional tests for titanium dioxide pigments as discussed above andsummarized in Table 1. The evaluations were performed using theoxidation base products from the Examples. Results for "control" refersto titanium dioxide product that was produced without the addition ofthe inert gas. The results for the examples of the present invention arelisted as "test".

In prior plant experience, it has been determined that CBU changes can,to a certain extent, affect the % >0.6 microns value. For example, ifCBU decreases about 1 unit, there is an increase in % >0.6 microns ofabout 2%. The parenthetical numbers shown for % >0.6 microns in Examples2A and 2B represent estimates for a CBU test condition equal that of thecontrol. Due to the limited duration of the plant test for Examples 2Aand 2B, there was not sufficient time during the test to maintain a CBUequal to the control.

                                      TABLE 1                                     __________________________________________________________________________              EXAMPLE                                                                       1  2A     2B     3A  3B 3C                                          __________________________________________________________________________    Production rate,                                                                        18 14     14     14  14 14                                          tons/hour                                                                     % >0.6 microns,                                                                         23.5                                                                             20.6   20.7   21.6                                                                              21.9                                                                             22.2                                        control                                                                       % >0.6 microns,                                                                         25.8                                                                             23.2                                                                             (25.4)                                                                            20.7                                                                             (23.1)                                                                            24.5                                                                              23.8                                                                             23.7                                        test                                                                          CBU, control                                                                            10.7                                                                             11.3   10.7   10.5                                                                              10.6                                                                             10.6                                        CBU, test 10.8                                                                             12.4                                                                             (11.3)                                                                            11.9                                                                             (10.7)                                                                            10.7                                                                              10.8                                                                             10.7                                        __________________________________________________________________________

Gloss change: control and test material from Example 1 were used toprepare a corresponding finished pigment grade by coating bothseparately with a thin alumina layer by appropriate surface treatmentfollowed by filtration, drying and micronization. The fractions >0.6micron for control and test were 7.9 and 8.4% respectively, and thecorresponding emulsion gloss levels were 67 and 64 respectively. Theseresults show that the test material was permanently made moreagglomerated by the procedure of this invention.

Having thus described and exemplified the invention with a certaindegree of particularity, it should be appreciated that the followingclaims are not to be limited but are to be afforded a scope commensuratewith the wording of each element of the claims and equivalents thereof.

We claim:
 1. A chloride process for producing TiO₂ comprising reactingat least vaporous TiCl₄ with an oxygen-containing gas to form a reactionmass in the presence of an inert gas injected into the reaction mass inan amount of about 0.005 to about 0.5 pounds per pound of TiO₂ and at atemperature of about -200° C. to about 1000° C. and at a pressure ofabout 500 to about 10,000 psig above reactor pressure.
 2. The process ofclaim 1 wherein the inert gas is introduced at one or more pointsdownstream of where the oxygen containing gas and TiCl₄ are initiallyreacted.
 3. The process of claim 2 wherein the inert gas is introducedat about 2 to about 200 feet after initial contact of theoxygen-containing gas and TiCl₄.
 4. The process of claim 3 wherein theinert gas is selected from the group consisting of nitrogen, chlorine,carbon dioxide, mixtures thereof, and gas mixtures rich in nitrogen,chlorine, carbon dioxide, and mixtures thereof.
 5. The process of claim4 wherein the inert gas is nitrogen and is added in an amount of about0.01 to about 0.1 pounds per pound of TiO₂ at about -20° C. to about200° C. at about 500 to 5000 psig above the reactor pressure.